1. Field of the Invention
This invention relates to an apparatus arranged to control a lens for varying the magnifying power of the lens or for focusing the lens.
2. Description of the Related Art
In a zoom lens of the kind having a focusing lens group disposed rearwardly of a (power) variator lens group, the position of the focusing lens group not only varies according to a distance to an object to be photographed (hereinafter referred to as object distance) but also varies with the position of the variator lens group, i.e., a magnifying power. In the case of a zoom lens of the kind having lens groups differently arranged in the order of a focusing lens group--a variator lens group --a compensator lens group--a relay lens group as viewed from the front side of the lens, the magnifying power of the lens can be changed while keeping it in an in-focus state by just varying the relative distances of the variator lens group and the compensator lens group in the direction of the optical axis of the lens, for example, by rotating a cam tube without changing the position of the focusing lens group. Unlike the zoom lens of that kind, in the zoom lens of the kind having focusing lens group disposed rearwardly of the variator lens group, the position of the focusing lens group must be changed according to the position of the variator lens group. In cases where shooting is made while changing the magnifying power of the lens, like in the case of a video camera, the picture must be prevented from blurring. The position of the focusing lens group must be continuously adjusted to changes taking place in the object distance through a complex process of lens position control.
A method for such zoom lens position control has been known, for example, from Japanese Laid-Open Patent Application No. SHO 62-296110.
This lens position control method is as follows: Referring to FIG. 4 of the accompanying drawings, the range of zoom positions of the variator lens group (on the axis of abscissa) from a telephoto end T to a wide-angle end W is equally divided into a plurality of zones. The locus of the focusing lens group for every one of these divided zones is stored in a memory. When the position of the variator lens group is shifted, information on the locus stored is taken out from the memory according to information on the position of the variator lens group. Then, the focusing lens group is moved along this locus.
Under the above-stated control, the information on the position of the variator lens group is detected by a position detecting means such as an encoder. Further, the information on the position of the focusing lens group which is arranged to be driven by means of a pulse motor is obtained by counting the number of the pulse motor driving pulses With the focusing lens group driven within a normal operable range of the pulse motor, the position of the focusing lens group is unconditionally determined by counting the number of the pulse motor driving pulses.
In the lens position control method, the position of the focusing lens group is controlled by determining whether or not the lens is out of focus in addition to the control performed according to the locus information obtained from the memory. In a suitable focus determining method, the light of the object is received through the zoom lens by a light receiving means which is composed of photoelectric conversion elements; and the focusing lens group is controlled in such a way as to bring the high-frequency component or the sharpness of an image signal output from the light receiving means to the peak value thereof, because: In cases where the object distance is externally measured by means of infrared rays or the like by the so-called active type method, the zoom lens in which the position of the focusing lens group varies with the zooming position of the lens (the position of the variator lens group) necessitates the positions of the focusing lens group corresponding to all the zooming positions of the lens to be stored in the memory, if the lens position control must be finely carried out. In that instance, the accuracy of distance measurement, the pitch of the divided areas, the zooming positions and the pitch of the focusing lens group must be very finely arranged. The necessary amount of information to be stored within a system increases to a great degree, thus necessitating the use of a memory of a large storage capacity.
Whereas, the focusing method of using an image signal does not necessitate use of a memory except for a zooming action. This method is capable of bringing the lens to an in-focus position without following the locus for every one of the object distances as shown in FIG. 4.
However, an arrangement to drive and control the focusing lens group in search of an in-focus position during a zooming process solely in accordance with the above-stated focusing method of using the image signal presents the following problem: In the case of a video camera, for instance, images are picked up at intervals of 1/60 sec (the NTSC system). In such a case, the speed of the focusing action is too slow for image sensing. To solve this problem, the focusing lens group is controlled by a focusing system by driving and controlling the lens group in accordance with the above-stated locus which is stored for every one of the plurality of divided zones.
More specific description of this is as follows: The zooming positions of the lens are divided into a plurality of zones as shown in FIG. 4. Within a range of near object distances, the loci of the focusing lens group relative to the object distance do not show much difference in the inclination of their curves. Therefore, the focusing lens group positions (the axis of ordinate of FIG. 4) are divided into a plurality of zones. The shifting speed of the focusing lens group is representatively determined for each of the divided zones and is stored in a memory. Then, the focusing lens group is driven at one of different representative speeds according to the information on the variator lens group position. Meanwhile, the blurred degree of an image thus obtained is periodically examined. When the blurred degree is found to have exceeded a given degree, the driving action on the focusing lens group at the representative speed is brought to a stop. Then, the focusing lens group is driven and shifted in the direction of reducing the degree of blur by increasing or decreasing the lens shifting speed. After an in-focus state is obtained, the lens group is again driven at the representative speed. The focusing action is thus finely performed without blurring the image by thus shifting the focusing lens group in close proximity to the locus. Further, before commencement of a zooming action, the focusing system is operated to bring the focusing lens group into an in-focus position, for example, when the power supply of the camera is switched on.
Therefore, this lens position control device is capable of efficiently controlling the focusing lens group with a memory of a very small storage capacity. The lens, therefore, can be driven and controlled with a very simple system.
In the case of the lens position control device which is arranged in this manner, on the other hand, the information on the position of the focusing lens group is obtained by counting the driving pulses of a pulse motor, if the focusing lens group is arranged to be driven by the pulse motor. Therefore, the focusing lens group must be in a position which is usable as a reference point for the pulse count. Information on the focusing lens group position is then obtained from a number of pulses counted when the focusing lens group is shifted to a certain position from the above-stated reference point. One of the divided zones (or areas) stored in the memory is then determined to be applicable from information on the focusing lens position and information on the variator lens group position. An applicable representative speed is then determined for a zooming action.
The focusing lens group can be set in a reference position (hereinafter referred to as a reset position) in varied manners, including: A method of bringing the focusing lens group back to a reset position when the power supply is switched off; and a method of clearing or resetting a focusing or zooming position storing memory after bringing the focusing lens group to the reset position when the power supply is switched on.
However, if the reset position which is to be used as reference for information on the position of the focusing lens group is set within a normal focusing lens group driving range, the focusing lens group might be caused to hit the reset position by a temperature error or a focusing error. This brings about some inconvenience in terms of load and system arrangement. In view of this, the reset position is set to the outside of the operating range of the focusing lens group.
Hence, when the power supply is switched on, the focusing lens group which is located in the reset position outside the operating range must be shifted from this position at the start of an automatic focusing action. As a result, a period of time required before an in-focus state is attained becomes long. In the case of a focusing method of detecting the peak of the high-frequency component of an image signal by using the image signal, in particular, the focusing speed becomes much slower than the focusing system of the external distance measuring type, because: In that case, if the focusing speed is too fast, the focusing lens group might be brought to a stop after overshooting the above-stated peak position. Further, since the reset position is set within an area beyond the infinity distance point of the focusing lens group shifting locus, the lens is in an extremely blurring state when it is in the reset position. This also causes the slow focusing speed.
Further, in cases where a lens state detecting device and a microcomputer or the like are used in combination for enhancing the operating efficiency of an automatic focusing system or the like by carrying out complex and precise lens position control, the resolving power of the above-stated lens state detecting device must be increased also. Such a device is typically represented by a lens position detecting device. Heretofore, the lens position detecting device of the kind having a high resolving power has often employed an encoder which is arranged to generate a digital signal in synchronism with the shift of the lens position.
Encoders which generate digital signals in the above-stated manner can be roughly classified into two kinds. One is called an absolute type encoder which is arranged to generate its own intrinsic signal corresponding to the position of the lens. The other is called an increment type encoder which is arranged to produce a number of pulses in proportion to an extent to which the position of the lens is shifted. In most cases, the increment type encoder is used for consumer appliances on account of the cost and size thereof.
However, with the conventional apparatus arranged in the above-stated manner, the use of the increment type encoder necessitates inclusion of a counter for counting the pulses of the encoder in the lens position control device which consists of a microcomputer, etc. Then, in order to detect an absolute position of the lens, the following operation processes are indispensable: The lens is shifted to a given reset position, for example, immediately after the power supply is switched on; and then the count position of the counter must be set at a value corresponding to the reset position of the lens before the counter is allowed to begin to count.
The position of the lens thus must be shifted always before the above-stated reset action when, for example, the power supply is switched on. This makes it impossible, for example, to have the apparatus in a standby state for shooting by switching the power supply off after setting the angle of view and a focal point.
Further, a method of forcedly shifting a phototaking lens simply inward when the power supply is turned off has been proposed by U.S. Pat. No. 4,827,296. However, the problem that an excessively long period of time is necessary before commencement of a normal operation after the power supply is switched on is hardly solvable by the method proposed.